1. Field of the Invention
The present invention relates to a structure of an inverter transformer employed in DC/AC inverter circuits for lightning an electrical discharge lamp. More specifically, the present invention relates to a structure of an inverter transformer which can be easily manufactured and in which leaking of magnetic flux from the joining portion of a pair of cores to the outside is prevented.
2. Description of the Related Art
A transformer in which a primary winding and a secondary winding are gripped on both sides by a pair of cores composed of a magnetic material and a prescribed electromagnetic coupling is conducted has been known in prior art as a transformer used in DC/AC inverter circuits for lightning an electrical discharge lamp employed as a back light for liquid crystal displays and the like. Such inverter transformers are disposed in narrow spaces and thin structures thereof are needed. In one example of the conventional transformers of this type, a structure is employed in which a primary winding is connected to an output terminal of an oscillation circuit (inverter) generating a high frequency and a secondary winding is connected to a discharge lamp via a ballast capacitor or the like, and in another example, a structure is employed in which part of the ballast capacitor is replaced with a choke coil.
A structure shown by a cross-sectional view in FIG. 1 is an example of an inverter transformer that has been typically used in prior art, this inverter transformer comprising a primary winding, a secondary winding, and a pair of cores arranged facing each other and forming a closed magnetic circuit, wherein electromagnetic coupling is formed by the primary winding, secondary winding, and cores. In FIG. 1, the reference numeral 101 stands for an E-type core, and 701, 601xe2x80x94the primary winding and secondary winding wound on side legs 103 of the E-type core.
The reference numeral 201 stands for an I-type core arranged facing the E-type core 101. A gap 202 is formed between a central legs 102 of E-type core 101 and the I-type core 201, and between the side legs 103 of E-type core and the I-type core 201. A spacer 203 composed of an electrically insulating material is usually sandwiched in the gap between the side legs 103 of E-type core and the I-type core 201, and the E-type core 101 and I-type core 201 form a closed magnetic circuit through the spacer 203. In the transformer of such a structure, magnetic resistance of the closed magnetic circuit can be changed and the degree of coupling of the primary winding 701 and secondary winding 601 can be adjusted by varying the thickness of spacer 203. However, in order to adjust the degree of coupling, spacers of different thickness have to be prepared and individually used according to the desired degree of coupling. As a result, not only the number of parts is increased, but also the assembly operation becomes complex. Another drawback is that because of the spacer thickness, the height dimension of the transformer cannot be reduced and a thin transformer is difficult to fabricate. Further, in case of the above-described structure, there is also a risk of the magnetic flux leaking from part of spacer 203 to the outside and magnetically affecting electronic components in the vicinity of the transformer or peripheral equipment.
Japanese Patent Application Laid-open No. H10-335157 disclosed an inverter transformer in which the adjustment of coupling coefficient of primary and secondary windings was facilitated, this inverter transformer comprising a pair of cores forming a closed magnetic circuit, a primary winding, and a secondary winding, wherein the lower core of the pair of cores is composed of a rectangular flat plate-like portion and two cylindrical projections integrally formed on both ends thereof, a bobbin winding shaft is attached to each of the projections, the primary winding and secondary winding are wound on the two winding shafts, and part of the primary winding is wound on the secondary winding.
Japanese Patent Application Laid-open No. 2000-124045 disclosed a structure of an inverter transformer for multiple discharge lamps, wherein a plurality of outputs are obtained with a single inverter transformer in which the first and second secondary windings with the same number of turns are arranged in positions centrally symmetric about the primary winding, and the primary winding and the first secondary winding, and the primary winding and the second secondary winding are electromagnetically coupled with almost the same degree of coupling.
All of those inventions relate to structures in which the primary winding and secondary winding are arranged in parallel. In the structure of the invention disclosed in Japanese Patent Application Laid-open No. H10-335157, part of the primary winding is wound on the secondary winding. Therefore, the lead-out wire of the primary winding, that is, the winding on the low voltage side is arranged so that it intersects with the secondary winding, that is, the winding on the high voltage side. As a result, there is a risk of degrading the transformer performance, unless the electric insulating properties in each winding are improved. Another drawback of those inventions is that there is a risk of the magnetic flux leaking to the outside from the joining portion of the cores.
On the other hand, in Japanese Patent Application Laid-open No. 2000-124045, the primary winding is arranged in the center and secondary windings are arranged on both sides thereof. The drawback of such a structure is that in the pairs of the primary winding and the first secondary winding, and the primary winding and the second secondary winding, the coupling coefficients between the primary winding and secondary windings are difficult to adjust.
Japanese Patent Application Laid-open No. 2000-68132 disclosed a structure of an inverter transformer with a shape suitable for installation in a space with large outside dimensions and a narrow width, wherein the lateral cross section of a winding shaft of a bobbin has an elliptical contour, the side with a narrow width faces the side surface to which the terminals of the base part of the bobbin are attached, the winding shaft is molded integrally with the base part, a central leg with an elliptical cross section is formed in the central part of one core, outer legs are formed at the four corners of the core, the central leg is inserted into the hole of the winding shaft, and the outer legs abut upon the other core. In the invention disclosed in Japanese Patent Application Laid-open No. 2000-68132, since the side surface portion of the core is in an open state, there is a risk of the magnetic flux leaking and the adverse magnetic effects being produced on peripheral electronic components or electronic devices.
Japanese Patent Application Laid-open No. 2000-243633 discloses a bobbin for decreasing the thickness of a transformer with a conventional structure in which a primary winding and a secondary winding are arranged above and below. Thus, this patent application discloses a bobbin for an inverter transformer, comprising a cylindrical winding shaft molded integrally with the upper surface of a base part, wherein a cylindrical second winding shaft concentric with the above-mentioned winding shaft is provided in a position is outside the above-mentioned winding shaft, and a hook projecting to the outside is formed at the upper end of the second winding shaft.
The invention disclosed in Japanese Patent Application Laid-open No. 2000-243633 relates to a structure in which a secondary winding is arranged at the central shaft and a primary winding is arranged concentrically on the outside. However, as shown in FIG. 3 of the specification relating to this application, in this invention, the primary winding is arranged so that it is exposed to the outside. As a result, there is a risk of the magnetic flux leaking to the outside and the adverse magnetic effects being produced on peripheral electronic components or electronic devices. Another drawback is that because the terminal is molded integrally with the bobbin, the operation of winding the windings on the bobbin is difficult to conduct.
It is an object of the present invention to provide an inverter transformer with a decreased height dimension and reduced thickness.
It is another object of the present invention to provide an inverter transformer which contains a small number of components and is easy to assemble.
It is yet another object of the present invention to provide an inverter transformer with no risk of the magnetic flux leaking to the outside from a joining portion of a pair of cores.
Still another object of the present invention is to provide an inverter transformer in which the degree of magnetic coupling between the primary winding and secondary winding can be easily adjusted.
The present invention provides an inverter transformer comprising a primary winding and a secondary winding arranged in the same plane and a pair of cores arranged facing each other and forming a closed magnetic circuit, wherein at least one of the cores of the pair of cores is constructed as a groove formation core. This groove formation core has a central leg and an outer perimeter wall and also has a partition between the central leg and outer perimeter wall, wherein groove portions are formed concentrically between the central leg and partition, and between the partition and outer perimeter wall, respectively. The primary winding and secondary winding are arranged concentrically in the respective groove portions of the groove formation core.
The other core may be a flat plate-shaped core or the above-described groove formation core. Thus, the pair of cores in accordance with the present invention may be a combination of the groove formation core and the plate-shaped core, or a combination of two groove formation cores.
In accordance with the present invention, the secondary winding is arranged in the groove portion between the central leg and partition of the groove formation core, and the primary winding is arranged concentrically with the secondary winding in the groove portion between the partition and outer perimeter wall.
Terminal plates composed of an electrically insulating material are provided on both side surface portions of the plate-shaped core arranged facing the groove formation core, and the primary winding and secondary winding are arranged in respective groove portions of the groove formation core. When the plate-shaped core is thereafter attached, the plate-shaped core is attached so that the outer perimeter wall of the groove formation core is tightly joined to the plate-shaped core and terminal plates. In a state in which the groove formation core and plate-shaped core are assembled, the inside of the groove formation core assumes a tightly sealed state. Further, at this time, a gap is formed between the flat-shaped core and the central leg and partition of the groove formation core.
In accordance with the present invention, the secondary winding can be formed by being divided into a main winding and a supplementary winding. In this case, the main winding is arranged in a groove portion between the central leg and partition of the groove formation core, and the supplementary winding is arranged concentrically with the main winding in the groove portion between the partition and outer perimeter wall via the partition. Furthermore, in the groove portion where the supplementary winding is arranged, the primary winding is arranged concentrically with the main winding and supplementary winding and adjacently to the outer side of the supplementary winding.
When the secondary winding is thus formed by being divided into the main winding and the supplementary winding, the main winding can be arranged in the groove portion upon winding on a bobbin, and both the supplementary winding and the primary winding can be arranged in a bobbin-less state in the groove portion. It is preferred that in this case the bobbin be provided integrally with the terminal plate.
In accordance with the present invention, since no spacer is used as means for forming a gap between a pair of cores in the inverter transformer, the height dimension of the transformer can be decreased. Further, since no spacer is required, the number of parts can be reduced, the assembly operation can be simplified, and productivity during fabrication can be increased.
Furthermore, in accordance with the present invention, a tightly sealed gap is formed by the outer perimeter wall of the groove formation core together with the plate-shaped core and terminal plates attached to the plate-shaped core. For this reason, the magnetic flux does not leak to the outside from the gap portion. Therefore, in accordance with the present invention, there is no risk of the leaking magnetic flux producing adverse magnetic effect on electronic components arranged around the inverter transformer or other electronic devices disposed around the device incorporating the inverter transformer.
Furthermore, in accordance with the present invention, the secondary winding is divided into a main winding and a supplementary winding, the main winding and the supplementary winding are arranged so as to be separated from each other by the partition, and the primary winding is arranged adjacently to the supplementary winding. Therefore, the degree of coupling of the primary winding and secondary winding can be easily adjusted by changing the number of turns in the supplementary winding.